This relates to cellular systems and, more particularly to space diversity in cellular systems.
Cellular networks provide service over a geographical area through cells that are each created by spatially separated non-mobile base stations. Each base station provides a communication channel to each wireless appliance (e.g., cell phone) in its area of coverage. The base stations interact with a network controller, which allows a cell phone in a cell that is served by one base station to communicate with cell phones served by other base stations, or to communicate with a landline phone served by a landline network. The network controller also assists in handoffs of a cell phone as it leaves the geographical area of one cell and enters the geographical area of another cell. In order to have good coverage, the base stations are often situated so that the areas covered by adjacent base stations (i.e., adjacent cells) overlap. Because of this overlap, in order to avoid interference problems initial designs of cellular networks assigned disjoint groups of channels to adjacent base stations.
In some of the newer systems the network controllers dynamically adjust the communication channels that the base stations may use. This allows a base station to use a channel that previously was used by an adjacent base station (for the benefit of some wireless appliance), but currently is not so used. The dynamic allocation of channels has three benefits. First, it generally increases the number channels that can concurrently carry conversations, second, it often allows a cell phone that has an active call to continue using the same channel even as it traverses from one cell to another, and third, it allows adjacent base stations to concurrently use a given channel as long as undue interference is not experienced, and only alter the channel assignments when interference is detected.
As depicted in FIG. 1 when cell phone 101 is in cell 10, which is the coverage area of base station 100, and employs a channel c1 that is assigned by base station 100, and cell phone 201 is in cell 20, which is the coverage area of base station 200, and employs a channel c2 that is assigned by base station 200, no interference can occur as long as channel c1 and c2 are distinct and the communication in channel c1 does not somehow spread to channel c2 (and vice versa). In fact, interference-free communication can take place even when cell phones 101 and 201 are on the same channel, as long as both cell phones are outside region 12. That is, as long as cell phone 101 cannot hear base station 200, and base station 200 cannot hear cellphone 101. It is noted that region 12 is the overlap area for communications between a wireless appliance, and two non-mobile base stations.
The overlap area between two wireless appliances where one is in cell 10 and the other is in cell 20 is larger than region 12 but, still, the interference region is significantly smaller than the area of cell 10 (or cell 20), so with the assumption that the location of a cell phone is random, one can note that the communication channels resource can be used more effectively by allowing adjacent channels to use the same set of channels, as long as a mechanism is provided for changing channel assignments so as to eliminate interference once it is recognized that interference is occurring, or information is available from which it may be deduced that interference is occurring. Illustratively, when cell phones 101 and 201 are assigned channel c1 and cell phone 101 travels toward region 200, enters region 12 and experiences interference, either cell phone 101 or cell phone 201 needs to be assigned a different available channel in order to remove the interference. The fact that cell phone 101 experiences base station interference is determined, for example, by cell phone 101 detecting pilot signals from both base stations 100 and 200, and sending the strength of the detected pilot signals through its communication with base station 100 to network controller 30. Based the received pilot signal strengths controller 30 can determine when interference is about to exceed permissible levels, prompting a reassignment procedure; and based on its knowledge of the availability of channels at base stations 100 and 200 network controller 30 can determine whether cell phone 101 or cell phone 201 should have its channel assignment altered (or whether, for some reason, both channel assignments should be altered), and informs the appropriate base station (or stations) accordingly. Controller 30 also determines whether a handoff should take place, assigning cell phone 101 to base station 200.